Generation of a stream of uniform drops has always been an important facet of ink jet printing systems. Uniformly sized drops may be charged and deflected uniformly and will form uniformly sized spots upon impacting a recording medium. Ink jet printing may be accomplished by one or a plurality of ink jet nozzles. An example of a multi-nozzle ink jet system is described in Sweet et al, U.S. Pat. No. 3,373,437, "Fluid Droplet Recorder with a Plurality of Jets." In that system, the jet nozzle orifices are arranged along a straight line and a recording medium is moved in a direction normal to that line while binary coded video signals are applied to selectively remove drops from the print streams. Two separate and alternative means of generating the drops are described. One means employs a magnetostrictive driver to vibrate the entire manifold including the nozzle orifices. This results in a velocity modulation of the streams. Another means includes a flexible wall for the manifold attached to the driver while the manifold is fixed in position to modulate the pressure of the fluid.
The ink is ejected from the nozzle orifices as continuous streams, the perturbations causing the streams to form varicosities which grow in amplitude until the continuous streams each break up into serial streams of uniformly sized drops.
The major problem with pressure modulation for a multinozzle ink jet with a common manifold is that the manifold cavity in which the pressure modulation occurs must not be too small such that the flow pattern behind the nozzles would differ from one to the other. Hence, when a larger manifold is used, the volume to be displaced to obtain proper pressure modulation is also increased substantially. Pressure modulation thus becomes less efficient.
On the other hand, velocity modulation requires no more displacement for multiple nozzles than for a single nozzle. However, the mass to be vibrated increases, reducing the efficiency. Further, when a flat nozzle plate is used, various resonances can result. It thus becomes difficult to maintain the plane of perturbation in a single row of nozzles in the same phase and in the same plane along the entire row. This may result in drop breakoff occurring at different times at different distances from the recording medium for the various streams in the row. Thus, where the recording medium is moving normal to the row of nozzles, as in Sweet et al, above, not all the drops would impact the recording medium at the same time to generate a straight line. Rather, a wavy or sloped line might result. Therefore, the structure must be designed with sufficient strength and mass to avoid adverse resonances, thereby further reducing the efficiency of the drop generator.
Stauffer, U.S. Pat. No. 3,334,351, "Ink Droplet Recorder with Plural Nozzle-Vibrators" describes the use of two separate transducers at different angles to impart dual motions to a single nozzle. The dual arrangement is manifestly inefficient. Further, when applied to a multi-nozzle head, the arrangement would result in a complex motion, making attainment of drop breakoff for all streams at the same distance from the recording medium extremely difficult.
Lyon et al, U.S. Pat. No. 3,739,393, "Apparatus and Method for Generation of Drops Using Bending Waves" describes vibration of one end of a nozzle plate to transmit bending waves to the other end of the plate which is damped, causing a velocity modulation of the jets. The structural design must therefore be carefully done and then manufactured under tight tolerances to operate at the desired frequency. Further, the modulation results in a phase delay between nozzles such that drops do not break off simultaneously.